In hover, each of the rotating blades of known rotorcraft rotors have the same airspeed, angle of attack and lift coefficient. Obviously, in forward flight, the blade rotating in the direction of the flight (advancing blade) will have its effective airspeed increased by the forward motion of the rotorcraft, i.e. effective velocity is equal to angular velocity plus vehicle velocity. Similarly, the blade rotating opposed to the direction of the flight (retreating blade) has its effective airspeed reduced by the vehicle velocity. (see FIG. 1).
Rotors generally comprise aerodynamically shaped blades attached to a rotating mast at the center of rotation. In some rotors (teetering rotors) the blades are attached to hub which is free to teeter one blade up, opposing blade down. In other rotors (articulated rotors) the hub is rigidly attached to the rotating mast and the blades are attached to the hub, at a point outboard of the center of rotation, by an articulated attachment allowing the blades flap up and down. In other rotors (semi-rigid rotors) the blades are attached to the rotating mast through a flexible hub which allows the blades to flap up or down proportionally to up or down moment applied by the blade on the hub. The blades of rigid rotors are rigidly attached to the hub and the rotating mast in the up-down flap direction. Semi-rigid and rigid rotors are also known as hingeless and bearingless rotors in the rotorcraft community
The moments applied by the blades and hub on the rotating mast are called mast moments. These moments are transferred to the rotorcraft airframe. The mast moment in the plane of rotation is the rotor driving torque, the mast moments out of the rotor plane of rotation are used for controlling the rotorcraft in roll and in pitch.
Teetering rotors, due to their teetering hinge, cannot apply out-of-plane mast moments to the airframe. Roll and pitch maneuvers of rotorcraft equipped with teetering rotors rely on tilting the plane of the rotor by using differential control of the blades' angle of attack (cyclic control). This causes the lift vector to tilt in the desired roll and pitch direction, resulting in the rotorcraft frame following the rotor maneuver lift vector, without mast moment having been applied.
Articulated rotors can apply out-of-plane mast moments proportional to the radial distance from the blade flapping hinge to the center of rotation. The trend in the development of such rotors was to increase such radial distance to enhance the roll and pitch maneuver response of rotorcraft equipped with such rotors.
Semi-rigid and rigid rotors apply out-of-plane mast moments proportional to the stiffness of their hub and blades in the up-down flap direction.
In co-axial rotor rotorcraft two rotors turn in opposing direction on the same mast. When such rotorcraft have rigid co-axial rotors, it is known to apply higher lift on the advancing blade of each rotor so that the mast moments on the two rotors cancel each other out. It is also known in such cases to increase the canceling mast moments as a function of advance ratio (the ratio of rotorcraft forward speed to the speed of the tip of the rotor due to rotational velocity). Such is the case in Sikorsky Aircraft's™ Advancing Blade Concept (“ABC”). Interestingly, however, beneficial application of opposing mast moments was never applied to rotor configurations with independent masts such as tandem rotors, side-by-side rotors, and tilt rotors.
The control mechanism of the prior art rotorcraft reduces the lift on advancing blade in forward flight to achieve zero roll mast moment. This is necessary in single rotor rotorcraft to avoid rolling the vehicle and in teetering rotors which are free to teeter (advancing blade up, retreating blade down). This zero mast moment approach results in the advancing blade having a lower lift coefficient and the retreating blade higher lift coefficient. At normal cruising speed, retreating blades of current rotorcraft (tilt-rotor aircraft in airplane mode excepted) are stalled from the center of rotation to a certain radius from the center due to the increase in local blade angle of attack beyond the stall angle of attack. This stall causes the rotor to have a decreasing total lift limit as the forward speed increases (FIG. 2). Furthermore, the stall of the retreating blade causes substantial rotor loads and rotorcraft vibration. Discussion of recent embodiments is contained in US 2005/0236518 to Scott (publ. Oct. 2005), the disclosure of which, along with any other extraneous materials referenced in the present application, is incorporated herein.
All current rotorcraft equipped with multiple lifting rotors (for example tandem rotor helicopters and tilt-rotor aircraft) have their rotors turn in opposing senses of rotation. But, the successful introduction of rotorcraft with a lifting rotor on each of multiple masts (tandem rotors, side-by-side rotors and twin tilt-rotors), and the introduction of rotors capable of out-of-plane mast moments (articulated, semi-rigid and rigid rotors), did not cause the rotorcraft industry to change from equalizing the lift of the advancing and retreating blades (some obvious use in unsuccessful coaxial rigid rotors excepted). Thus, there is still a need to apply opposing mast moments to rotors on independent masts such as tandem rotor, side-by-side, and tilt rotors.